Compton backscatter imaging (CBI) is a single-sided imaging technique in which the radiation source and the detection/imaging device are located on the same side of the object. As a result, CBI is a valuable non-destructive inspection (NDI) tool because of its single-sided nature, the penetrating abilities of radiation, and unique interaction properties of radiation with matter. Changes in the backscatter photon field intensity (resulting in contrast changes in images) are caused by differences in absorption and scattering cross sections along the path of the scattered photons. Since the inception of CBI, a diverse set of imaging techniques have evolved using both collimated and un-collimated detectors, coded apertures, and hard x-ray optics. “Pencil beam” CBI uses a highly collimated beam of radiation to interrogate objects. The pencil beams may vary in diameter from microns to centimeters, but usually consist of a near-parallel array of photons forming a tight beam. A common implementation uses rotating collimators to sweep a pencil beam across an object in an inspection area. A detector measures the backscatter from the CBI pencil beam as it scans the object. One or more CBI pencil beams may be used in combination with relative motion between the object and the CBI beams (the relative motion being generally orthogonal to the plane of the sweep) to raster scan an object and construct one or more 2-D images of interior portions of the object.
When inspecting objects such as cargo containers or vehicles with CBI it is often desirable to construct a 3-D image of the object. This requires scanning the object from multiple perspective views. In order to speed up the inspection process it is desirable to use two or more CBI beams, each with its own detector, to interrogate the object from multiple perspectives. However, if the CBI beams are operating concurrently, a problem that may occur is cross-talk between the CBI/detector systems. That is, backscatter from one CBI beam may cause noise (a signal that is spatially irrelevant) in another CBI beam's detector. One approach to avoid this problem is to time-phase the CBI beams. So, for example, if three rotating CBI collimators are used the radiation source associated with each collimator being used only one third of the time, which is an inefficient use of that device. What are needed therefore are systems for CBI that minimize cross-talk and improve efficiency.